1. Field of the Invention
The invention relates in general to a thin film transistor (TFT) structure and the method of manufacturing the same, and more particularly to a TFT structure with higher quality and a manufacturing method thereof with less photolithographic steps.
2. Description of the Related Art
The thin film transistor (TFT) has been widely used in liquid crystal displays (LCD), wherein a conventional manufacturing method thereof is shown in FIGS. 1 to 5.
FIG. 1 illustrates the formation of a gate on a substrate. First, a substrate 100 is provided, and a metal layer is deposited on the substrate 100. Then, a gate electrode 110 is formed by photolithography and etching the metal layer.
FIG. 2 illustrates the formation of a gate insulation layer, an amorphous silicon layer, and an ohmic contact layer. A gate insulation layer 120 is deposited on the substrate 100, covering the gate electrode 110. Then, an amorphous silicon layer and an ohmic contact layer are formed sequentially on the gate insulation layer 120, and are etched using a patterned photoresist (not shown) as a mask. The patterned photoresist is then removed and therefore a patterned amorphous silicon layer 130 and an ohmic contact layer 140 are formed as shown in FIG. 2.
FIG. 3 illustrates the formation of a source electrode and a drain electrode. A metal layer is formed on the substrate 100 and is patterned by a lithography and etching step. An opening 162 is formed in the metal layer, to open through the patterned ohmic contact layer 140 and to expose portion of the amorphous silicon layer 130. A source electrode 165 and a drain electrode 160 are consequently formed while forming the opening 162.
FIG. 4 illustrates the formation of a passivation layer on the substrate 100. A passivation layer 170 is deposited on the substrate 100. Then, an opening 172 is formed by a photolithography and etching process to expose the drain electrode 160.
FIG. 5 illustrates the formation of a transparent electrode. A transparent conductive layer is deposited on the passivation layer 170 and fills the opening 172 of the passivation layer 170. The transparent conductive layer is patterned by lithography and etching methods; a transparent electrode 180 is formed, consequently. Five masks are required in the TFT manufacturing processes described above, which are respectively shown in FIGS. 1 to 5.
A manufacturing method using four masks to fabricate a thin film transistor has been developed by combining the steps of FIG. 2 and FIG. 3. However, the light incident from the rear side of the substrate directly illuminates the amorphous silicon layer, and a photo-induced current is undesirably produced. Therefore, the OFF characteristic of the TFT becomes worsen, and result in deterioration of the quality of the TFT is deteriorated.